The present invention relates to a process for producing an "optical circuit". In a general manner, this term refers to a device having semiconductor layers able to produce an amplifier medium, a means for producing an optical resonator, a means for extracting the stimulated optical radiation from the amplifier medium and guide it towards the outside and various other components, such as photodetectors or electrooptical modulators. Such devices are used in optical telecommunications.
The invention aims at obtaining such a circuit in a monolithic integrated version, i.e. which calls on a common technology for producing the various components and does not involve the assembly of various separately produced components.
One of the difficulties caused by the monolithic integration of a semiconductor laser is the production of the resonator which, combined with the active layer forming the amplifier medium, makes it possible to obtain a coherent light radiation. Several solutions have been adopted in the prior art, such as producing two mirrors by ionic or chemical working or machining, the use of periodic structures of the diffractive grating type, placed outside the active layer or distributed therein, etc.
Another problem caused by monolithic integration is the necessity of having a transparent layer, which can serve as an optical guide to the radiation emitted by the active layer. The coupling between the active layer and the optical guide leads to a redefinition of the laser structure.
The attached FIGS. 1 to 4 give further details on these problems. They describe several known structures offering different solutions for these problems.
The laser shown in FIG. 1 comprises a substrate 10, a first confinement layer 11, an active layer 12, a second confinement layer 13, a contact layer 14, a metal layer 15 forming the first ohmic contact and a metal layer 16 positioned beneath the substrate and forming the second ohmic contact. A transparent insulating layer 17 covers the edges of the structure. An optical waveguide 18 is placed on either side of the laser. The composition and thickness of the main layers can e.g. be as follows:
______________________________________ 15 Cr--Au 14 p.sup.+ GaAs 1 .mu.m 13 p.sup.+ Al.sub.0.3 Ga.sub.0.7 As 2 .mu.m 12 p.sup.+ GaAs--Si 1 .mu.m 11 n.sup.+ Al.sub.0.3 Ga.sub.0.7 As 2.5 .mu.m 10 n.sup.+ GaAs 16 Au--Sn 17 SiO.sub.2 18 n.sup.- GaAs 12 .mu.m ______________________________________
This known structure is described in the article by C. E. Hurwitz et al entitled "Integrated GaAs-AlGaAs double-heterostructure lasers", published in Applied Physics letters, vol. 27, no. 4, 15.8.1975, pp. 241/3.
The structure shown in FIG. 2 also comprises an optical waveguide coupled to the active layer, but which is arranged in the manner of so-called "Integrated Twin-Guide Lasers with Distributed Bragg Reflector" or in abbreviated form "DBR-ITG Lasers".
As shown the laser comprises a type n InP substrate 30, a layer 28 of Ga.sub.u In.sub.1-u As.sub.v P.sub.1-v (u and v&lt;1), a type n InP confinement layer 26, an active layer 31 of Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y (x and y&lt;1), a type p GaInAsP layer 32, a type p InP layer 35, a GaInAsP contact layer 36 and finally a gold layer 40. The diffractive grating is formed on the separating layer 26. The electrical insulation is provided by the SiO.sub.2 layer 24.
Such a structure is more particularly described in the article by K. Utaka et al entitled "1.5-1.6 .mu.m GaInAsP/InP Integrated Twin-Guide Lasers with First Order Distributed Bragg Reflectors" published in Electronics Letters, 5.6.1980, . vol. 16, no. 12.
The refractive index of the layers is shown in the left-hand part of the drawing. It can be seen that the index of layer 28 is higher than the indices of the adjacent layers 26 and 30, which is favourable for the guidance of the radiation in layer 28.
As is also known the diffraction grating of the laser can be distributed along the amplifier medium instead of being placed at each of its ends, which leads to a distributed feedback or DFB laser.
Such a device is shown in FIG. 3. It comprises a type n GaAs substrate 70, a type n Ga.sub.0.7 Al.sub.0.3 As layer 72, a type p GaAs active layer 74, a type p Ga.sub.0.8 Al.sub.0.2 As layer 76 surmounted by a type p Ga.sub.0.93 Al.sub.0.07 As layer 78 where the distributed grating is formed, a type p Ga.sub.0.7 Al.sub.0.3 As layer 80, a Ga.sub.0.9 Al.sub.0.1 As layer 81 type p-doped by Zn diffusion above the active layer and a conductive layer 83. Radiation is removed by a waveguide incorporating a type p Ga.sub.0.7 Al.sub.0.3 As layer 82 surmounted by a Ga.sub.0.9 Al.sub.0.1 As layer 84, which is not doped outside the active layer of the laser.
Such a structure is described by K. Aiki et al in an article entitled "A Frequency Multiplexing Light Source with Monolithically Integrated Distributed-Feedback Diode Lasers", published in IEEE Journal of Quantum Electronics, vol. QE-13, no. 4, April 1977. pp. 220-223.
Finally, FIG. 4 shows an integrated optical device incorporating a laser 90 and a photodetector 92, each of these components essentially having an identical active layer, respectively 91 and 93. These layers are optically coupled to an optical guide 94, in accordance with the principles described hereinbefore. Such a structure is described in the article by J. L. Merz et al entitled "GaAs Integrated Optical Circuits by Wet Chemical Etching", published in IEEE Journal of Quantum Electronics, vol. QE-15, no. 2, February 1979, pp. 72-82.
All these methods suffer from disadvantages, either in that they lead to a mediocre coupling between the active layer and the optical guide (variants of FIGS. 2 and 4) or in that they require several epitaxy operations (variants of FIGS. 1 and 3).